A spokeswoman for the Middlesex District Attorney's Office said Sunday that the medical examiner concluded that acute cyanide toxicity killed Stephen Rakes, 59, of Quincy in July. His death was ruled a homicide.

Rakes learned before his death that he wouldn't be called as a witness against Bulger, the Boston gangster convicted of murder in August.

Authorities say Rakes' death wasn't related to the Bulger case. Rakes' business associate, 69-year-old William Camuti of Sudbury, was charged with attempted murder and other crimes in Rakes' death and has pleaded not guilty.

Officials say they will now seek additional charges against Camuti.

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